The specific salient objectives of this research are: 1. To determine if imipramine treatment blocks the panic provoked by intravenous lactate in patients with Panic Disorder, under controlled single blind circumstances; 2. To investigate the biochemistry and psychophysiology of the panic attack and in particular the antecedents of the attack; 3. To investigate whether lactate produces this specific effect by its action on acid base balance and calcium states; 4. To determine if the improvement during imipramine treatment is related to blockade of lactate induced panic; and 5. To determine if the improvement maintained after imipramine for one month is related to blockade of lactate induced panic under double-blind drug vs. placebo conditions.